The Draugen Platform and Subsea Structures, Installation and Foundation Aspects

Abstract

ABSTRACT Installation data from the Draugen GBS and subsea structures have been processed and analysed in order to verify the design assumptions with regards to soil reactions. These results have shown that penetration ofconcrete skirts and piles into the very hard clay layer, typically found at the Haltenbanken area is feasible. The resistances may be predicted with reasonable accuracy using conventional methods and average soil strength values as obtained by traditional soil testing. This paper reveals and comments the installation data both for the gravity platform and the piles for the different subsea structures and provides and updated basis for foundation design of similar structures in the area. THE DRAUGEN FIELD The Draugen field is located west of mid Norway in The Haltenbanken area, and is the first oilfield being developed in this region. This area have seabed topography and soil conditions rather unlike those commonly encountered in the North Sea. The heart of the field installations is the concrete Gravity Base Structure, GBS, founded on the seabed at approximately 250 meters depth of water. This structure was at time of installation, the tallest concrete structure ever installed. A number of different subsea structures are connected to the GBS through flexible flowlines. There are two water injection templates, each housing 5 injection wells. In addition, there are two single oil production wells, and one single gas injection well, each protected by separate wellhead protection structures, WPS. The oil produced is centrally stored and processed at the GBS, and exported to an offshore loading unit through two large steel pipe lines and flexible risers. At the connections between the steel pipes and flexible lines, a pipeline end manifold, PLEM, is installed. All of these subsea structures were founded by steel pipe piles. An artistic impression of the field installations is shown on Fig. 1.(Available in full paper). The Draugen GBS platform was installed in May 1993, andthe subsea structures were installed during the following months. SEABED AND SOIL CONDITION In general, the seabed conditions at the Draugen area is characterised by a rather uneven seafloor, which is in contrast to all other areas in the Norwegian sector where large platforms have been installed. The area is crossed by many ploughmarks from the latest glacial period, and locally, there are areas on the seafloor where boulders and corals occur rather frequently. Boulders of sizes up to 1 meter in equivalent diameter have been observed. At the GBS site, the seabed is relatively even, with water depths ranging from 251 to 254 meters. The water depth increases towards south. Contours for each 25 cm of the seabed at the platform location area, is presented on the seabed map shown on Fig. 2. This map is based on the use of a multibeam echosounder and proved to be very accurate with regards to local details. The soil conditions at the GBS site consists of an upper 6 to 7 meter thick layer of silty, sandy, very soft to medium clay, with gravel and occasionally cobbles.